Question

Postgresql中最好的方法是什么，我想计算字段A中value = 1计数的数量以及同一字段A中value = 0计数的数量。

类似的东西：

select 
    count (field1 = value1) as filed1_ok, 
    count (field1 = value2) as filed1_bad,
    extract(MONTH from rpt_date) AS mth
where rpt_date between frdate and todate
group by mth

Answer 1

使用count(condition or null)

select
    count(field1 = 1 or null) as filed1_ok,
    count(field1 = 0 or null) as filed1_bad,
    extract(MONTH from rpt_date) AS mth
where rpt_date between frdate and todate
group by mth

true or null评估为true。 false or null评估为null。由于count不计算您想要的空值。

在其他SQL方言和Postgresql中，可以使用case

完成

select
    coalesce(sum(case field1 when 1 then 1 end), 0) as filed1_ok,
    coalesce(sum(case field1 when 0 then 1 end), 0) as filed1_bad,
    extract(MONTH from rpt_date) AS mth
where rpt_date between frdate and todate
group by mth

与count(condition or null) Postgresql选项相比，我认为它冗长且不透明。

PostgreSQL计数（布尔表达式）

1 个答案: